It is known that CD-Rs (CD-Recordable) have been proposed and developed as write-once-read-many optical recording media which comply with standards of compact discs (hereinafter, abbreviated to “CD”), and they are widely utilized for music replay media and information terminals.
For recording and replaying such an optical recording medium, a near-infrared semiconductor laser of 770 to 830 nm is generally used, and signal recording is carried out in a heat mode onto a recording layer comprising an organic dye and so forth on a substrate. Namely, when the recording layer is irradiated with a laser light, the organic dye absorbs light to generate heat and a pit is formed in the recording layer by the generated heat. Then, the recorded signal is detected on the basis of reflectance, at the irradiation with the laser light, of a portion where the pit has been formed and a portion where the pit is not formed.
Since satisfying CD standards such as Red Book and Orange Book, the CD-R has a characteristic that it is compatible with a CD player and a CD-ROM player. However, the recording capacity of the above conventional medium is about 680 MB, which is not sufficient for the recording of motion pictures. Therefore, with the rapid increase of information volume, the demand of a high-density and large-capacity information-recording medium increases.
A measure for obtaining the high-density recording medium comprises the reduction of a beam spot size by shortening the wavelength of the laser for use in recording and replaying and by increasing numerical apertures (N.A.) of the objective lens. Thus, short-wavelength lasers having a wavelength such as 680, 670, 660, 650, or 635 nm, which are utilized in an optical disc system, have been put into practical use. Accordingly, it becomes possible to form an optical recording medium capable of recording motion pictures and a large volume of information by shortening of the wavelength of a semiconductor laser, increase of Numerical Aperture of the objective lens, data-compacting technology, and the like. As a result, developed as a write-once-read-many optical recording medium applicable to the above lasers is a recordable Digital Versatile Disc (hereinafter, abbreviated as DVD-R). DVD-R is an optical recording medium having a recording capacity of 3.9 GB or 4.7 GB and capable of writing once, and the development of an optical disc having a good recording property applicable to such a capacity is further desired. The wavelength of the red laser to be used at that time is from 550 to 700, preferably 635 to 670 nm. In this situation, proposed optical recording media include magneto-optical recording media, phase change recording media, chalcogen oxide-type recording media, organic dye-type optical recording media, and the like. Of these, the organic dye-type optical recording media are considered to be advantageous in view of inexpensiveness and easiness of processing.
Recordable optical recording media wherein a dye is used as the recording layer and a reflective layer is provided on the recording layer for the purpose of increasing reflectance have widely been commercialized as write-once-read-many compact disk (Compact Disc Recordable: CDR) media using a cyanine dye or a phthalocyanine dye as the recording layer since the former medium was disclosed in Optical Data Storage 1989 Technical Digest Series Vol. 1, 45 ('89). These media have characteristics that they can be recorded with a semiconductor laser of 780 nm and replayed on commercially popular and commercially available CD player and CD-ROM player.
In addition, recently, as an optical recording medium capable of recording and replaying motion pictures having a density higher than CD and a quality similar to TV, a DVD-R medium having a capacity of 4.7 GB at one side has begun to be supplied commercially, the media being capable of recording with a red semiconductor laser having an oscillation wavelength of 635 to 660 nm and of replaying on commercially available DVD video player and DVD-ROM player which has begun to be popular. The DVD-R medium also uses a cyanine dye or an azo dye as the recording layer and employs a laminated structure with a reflective layer, the medium being characterized in a disc structure wherein two sheets of substrate having a thickness of 0.6 mm are laminated.
Furthermore, a higher density recording may be required in future, and the volume of information is expected to reach as much as from 15 to 30 GB. As a means for realizing such a recording density, it is inevitable to use a shorter-wavelength laser. Therefore, as a recording dye for use in an organic dye-type optical recording medium in future, a dye having a good recording property in the wavelength range of 300 to 500 nm is desired.
By the way, with regard to the medium capable of recording at a density higher than DVDR wherein an organic dye is used as the recording layer, Japanese Patent Application Laid-Open No. 302310/1998 discloses that a density of a recording capacity of 8 GB or more is achieved using a laser having an oscillation wavelength of 680 nm or less. In the proposal of the publication, a large recording capacity of 8 GB or more is attained by converging a laser light of 680 nm or less by means of an objective lens having a high Numerical Aperture of 0.7 or more through a light-transmitting layer having a thickness of 10 to 177 μm.
On the other hand, in these years, as blue lasers, a laser of 410 nm using a GaN-type material and an SHG laser having a wavelength of 425 nm by the combination of a semiconductor laser and an optical waveguide device have been developed (e.g., Nikkei Electronics No. 708, p. 117, Jan. 26, 1998), the development of dyes applicable to a blue semiconductor laser corresponding to such lasers is now in progress.
Furthermore, from the beginning of 1999, GaN-type semiconductor lasers having an oscillation wavelength of 390 to 430 nm has been offered as samples (Nichia Corporation), and thereafter, it has begun to study a medium (hereinafter, referred to as HD-DVDR) having a highly dense capacity of 15 GB or more at one side capable of recording about 2 hours of motion pictures having a picture quality similar to that of HDTV (high definition television) broadcast. When the HD-DVDR medium having such a highly dense capacity is used, about 6 hours of recording is possible for the picture quality similar to that of present broadcast, and therefore, the medium has also attracted the attention as a new recording media which will replace home VTR. Already, as a proposal of using a phase change inorganic recording film, the technical summary is introduced in Nikkei Electronics (No. 751), p. 117, Sep. 6, 1999.
Heretofore, as examples of dyes recordable exclusively with a blue laser of 400 to 500 nm, there have been reported cyanine dye compounds described in Japanese Patent Application Laid-Open Nos. 74690/1992 and 40161/1994 and porphyrin dye compounds described in Japanese Patent Application Laid-Open Nos. 304256/1995, 3042567/1995, 127174/1996, and 334207/1999, and also polyene dye compounds described in Japanese Patent Application Laid-Open Nos. 78576/1992 and 89279/1992, azo dye compounds described in Japanese Patent Application Laid-Open Nos. 334204/1999 and 334205/1999, dicyanovinylphenyl dye compounds described in Japanese Patent Application Laid-Open No. 334206/1999, and the like.
Moreover, there have been proposed an optical recording medium described in Japanese Patent Application Laid-Open No. 53785/1999 which comprises two layers of a recording layer using mainly a porphyrin dye, a cyanine dye, or the like as an organic dye for recording layer formation and a metal reflective layer mainly composed of silver, an optical recording medium having an ingenious medium constitution described in Japanese Patent Application Laid-Open No. 203729/1999 which enables the recording at two wavelength regions by including a blue laser-sensitive dye layer containing a cyanine dye sensitive to a blue laser and a red laser-sensitive dye layer, an optical recording medium using indigo dye compounds described in Japanese Patent Application Laid-Open No. 78239/1999 which enables the recording at two wavelength regions by mixing two kinds of dyes, i.e., a dye for a blue laser and a dye for a red laser, an optical recording medium using a cyanoethene dye described in Japanese Patent Application Laid-Open No. 105423/1999, an optical recording medium using a squalilium dye compound described in Japanese Patent Application Laid-Open No. 110815/1999, and the like.
On the other hand, as an example of recording onto an organic dye film at a blue region of 400 to 500 nm, Japanese Patent Application Laid-Open Nos. 304256/1995 and 304257/1995 describe a proposal which aims at the reduction of production cost by mixing with a polymer having a molecular structure coordinating to the central metal of a porphyrin compound at the side chain to shift the Soret band of the porphyrin compound to a longer-wavelength side, whereby the dye is made applicable to Ar laser of 488 nm and also by enabling the film formation by spin coating. Moreover, the polyene compounds disclosed in Japanese Patent Application Laid-Open Nos. 78576/1992 and 89279/1992 and so forth are found to have a poor stability to light according to the examination carried out by the present inventors, and thus some modifications such as blending of a quencher is necessary for it practical application.
Furthermore, as an optical recording medium capable of recording with lasers having both wavelength regions, there is an optical recording medium using a porphyrin compound described in Japanese Patent Application Laid-Open No. 101953/1998 or an optical recording medium using a tetraazaporphyrin dye compound described in Japanese Patent Application Laid-Open No. 144312/1999. Namely, porphyrin compounds and azaporphyrin compounds having a similar structure have a characteristic that they have absorption called “Q band” at a longer-wavelength side of the visible region and strong absorption also at a shorter-wavelength region of the visible region called “Soret band”. Cyclic organic compounds such as porphyrin widely used as dyes, pigments, photoelectric functional materials, and the like as the uses are proposed as compounds having properties suitable for dyes for DVD-R and also for dyes for optical recording media capable of a high density recording applicable to 15 to 30 GB.
The process for producing azaporphyrins is precisely described in The Porphyrines, Vol. 1, pp. 365-388, edited by D. Dorphine, Academic Press, but as general synthetic methods, known are processes for producing tetrabenzodiazaporphyrins and tetrabenzotriazaporphyrins by heating acetophenone and phthalonitrile in a metal halide as described in Annalen, 1937, Vol. 529, p. 205, Annalen, 1937, Vol. 531, p. 279, and so forth and processes for producing substituted diazaporphyrins, in which no aromatic group is fused, by dimerizing a pyrromethene derivative as described in J. Chem. Soc. (C), 1996, pp. 22-26 and J. Biochem., 1997, pp. 654-660.
As a recent situation, since a blue-violet semiconductor laser having a wavelength of 400 to 410 nm is well on the way to put into practical use, a large-capacity write-once-read-many optical recording medium using the laser has actively developed and therefore, the development of a dye having a high light resistance and a good high-speed recording property is particularly desired.
However, it is a present situation that the above optical recording medium for a blue semiconductor laser is not sufficiently applicable to a laser light having a wavelength of 400 to 410 nm. Namely, the present inventors have found problems that a replay on the medium using the above organic dye does not work well because a good value of the ratio of carrier wave to noise (C/N) is not necessarily obtained at the replay of the recorded signals, photodeterioration by the replay light occurs owing to an insufficient light resistance, and thus a high-grade signal property is not necessarily obtained. Accordingly, it becomes an urgent necessity to overcome the problems and to develop an optical recording medium capable of a high-density recording and replay with a laser light having a wavelength of 400 to 410 nm. Furthermore, in the case of applying to DVD-R having a capacity of 4.7 GB which is strongly desired as a recording and replaying medium for digital motion pictures, it is inevitable to contain an organic dye sensitive to a laser having a wavelength of 635 to 670 nm in the recording layer of the medium.
For the purpose, the object cannot be achieved by the above recording dye alone which is used exclusively for a blue laser wavelength.
In addition, the optical recording medium described in Japanese Patent Application Laid-Open No. 203729/1999, which is capable of the recording and replay at two wavelength regions of a blue laser wavelength region and a red laser wavelength region, has a multilayered recording layer and the optical recording media described in Japanese Patent Application Laid-Open Nos. 78239/1999, 105423/1999, and 110815/1999 necessarily uses two or more kinds of recording dyes, so that the preparation of the media is complicated and there is yet room for improvement of the recording property. Moreover, with regard to the optical recording media described in Japanese Patent Application Laid-Open Nos. 101953/1998 and 144312/1999, satisfactory property is not sufficiently obtained at the recording and replay with each laser light selected from both the wavelength ranges of 400 to 410 nm and 635 to 670 nm, and thus, at present, there is room for improvement of the recording dye compounds used for the optical recording media of the publications.
The present inventors have examined recording materials suitable for write-once-read-many optical recording media and obtained the following three findings.    (1) A large-capacity write-once-read-many optical recording medium uses a laser light of 300 to 500 nm and/or a laser light of 500 to 700 nm for writing and reading records, so that the control of absorption coefficient, refractive index, and reflectance at around the wavelength of the laser lights is important as a recording material.    (2) Although large-capacity write-once-read-many optical recording media using the laser lights have been actively developed as mentioned above and, in particular, a dye having a high light resistance and a good high-speed recording property has been desired, the above dye compounds do not yet have a sufficient property as recording materials capable of the recording and replay with the laser lights at both wavelength regions, and therefore there is room for improvement at present. Moreover, at the production of the media by coating methods such as spin coating which are convenient for the formation of recording films, a high solubility in a coating solvent is mentioned as one advantageous property and thus it is necessary to consider this point.    (3) With regard to the above processes for producing azaporphyrin compounds, no diazaporphyrin described in the present invention is obtained by the method of heating an acetophenone and a phthalocyanine in a metal halide. Also, neither α,β-azaporphyrin nor trisazaporphyrin described in the present invention is obtained by the production method of the dimerization of a pyrromethene derivative.
Moreover, in general, it is necessary to carrying out a higher-density recording for increasing the recording capacity, so that it is inevitable to increase Numerical Aperture of the objective lens used for converging the optical beam for recording and to use a laser having a shorter wavelength in the optical system. However, the minimum beam diameter of the converged optical beam is decided by the diffraction limit.
By the way, since the recording is carried out at a beam strength exceeding a certain threshold value, a recording pit smaller than the converged beam spot is obtained as shown in FIG. 7(a). The surrounding area of the recording pit corresponds to the foot of the strength peak of the beam. Under the present circumstances that a shorter-wavelength beam is increasingly used, photochemical reaction of the recording layer is promoted even at the surrounding area of the recording pit. In particular, there is a problem that the pit edge is degraded at the recording and thus signal property deteriorated because the wavelength region of the above blue-violet laser is a wavelength region at which photochemical reaction of an organic compound easily occurs. That is, as shown in FIG. 7(b), a recorded information (solid line part in FIG. 7(b)) which should be, by right, formed so as to correspond to rectangular wave becomes a broader waveform (broken line part in FIG. 7(b)). Additionally, when the information is replayed at the wavelength of the same blue-violet laser, there arises a problem that photoreaction is promoted even by the irradiation with a weak light such as a replaying light and thus the degradation proceeds every time replayed. Therefore, in Japanese Patent Application Laid-Open Nos. 304256/1995, 304257/1995, and so forth, a countermeasure of differentiating the wavelengths of the recording light and the replaying light, actually using a replaying light having a wavelength longer than that of the recording light should be taken and, as a result, it is an actual situation that the requirement for a sufficient high-density recording cannot be satisfied. Furthermore, the differentiation of the wavelengths of the recording light and the replaying light needs preparing a recording apparatus and a replaying apparatus separately or providing two optical systems and control systems thereof in one apparatus, which results in limitation of the use as an optical recording medium or invites enlargement of the apparatus and increase of cost, and finally the medium becomes poor in generality. Moreover, hitherto, in the optical recording media such as CDR, ON or OFF of the recording is carried out according to a definite thermal threshold value of a physical property such as melting point, sublimation point, phase transition point, or thermal decomposition point of an organic dye film, but the intervention of the photodeterioration mode caused by the excitation with a blue-violet laser makes the contrast indefinite and especially in the high density recording system wherein a fine recording pit smaller than an optical beam should be formed, there is a risk of remarkable deterioration of quality of a recorded signal.